Many approaches have been taken to effect the removal and/or decomposition of nitrogen oxides (NO and NO.sub.2 ; together referred to herein as "NO.sub.x ") from gaseous mixtures containing them. Nitrogen oxides surface as pollutants in a wide variety of contexts. Primarily, nitrogen oxides appear as pollutants in combustion processes; thus, they are present in coal, natural gas and diesel combustion, vehicular exhaust, and various refining and industrial manufacturing applications. Currently, NO.sub.x emission is estimated to be at least 20 million tons per year, with combustion of fuel the main source of NO.sub.x generation. To date, however, the various solutions which have been proposed to deal with NO.sub.x pollution have been unsatisfactory in one respect or another.
U.S. Pat. No. 4,659,448 to Gordon summarizes several of the approaches which have been taken to address NO.sub.x pollution. One approach is that described by R. Mahhaligam et al. in "Catalysts Development and Evaluation in the Control of High-Temperature NO.sub.x Emissions", The American Institute of Chemical Engineers 77(211):9-25 (1981), which involves passing gas mixtures containing NO.sub.x through a heated nickel and cobalt catalyst bed, contained in a ceramic tube. Other approaches cited in the Gordon patent include the following: that described by S. Pancharatram et al., in "Catalytic Decomposition of Nitric Oxide on Zirconia by Electrolytic Removal of Oxygen", Journal of Electrochemical Society 122:869-875 (1975); that described by E. F. Sverdrup et al. in "Design of High-Temperature Solid-Electrolyte Fuel-Cell Batteries for Maximum Power Output per Unit Volume", Energy Conversion 13:129-141 (1973); and the catalytic dissociation apparatus of U.S. Pat. No. 4,253,925 to Mason. This latter reference describes use of a "stabilized" oxygen-ion oxide as a solid electrolyte in the form of a film. A gaseous stream is passed along the cathodic surface of the electrolyte while the anodic surface is exposed to air; nitrogen oxides contained in the gaseous stream catalytically dissociate at the cathodic surface. The process is conducted at a temperature in the range of 400.degree. C. to 1000.degree. C. while applying a unidirectional electric field through the electrolyte by means of a direct current voltage source. The Gordon patent involves use of a system similar to that described in the Mason patent.
The present invention is directed to a new system which provides for a number of advantages relative to the systems of the prior art. First, the invention provides for selective adsorption of NO.sub.x from a gaseous stream containing a mixture of gases; that is, a sorbent material is provided which preferentially adsorbs NO.sub.x relative to other gases such as oxygen. Second, the sorbent material used is such that formation of thermodynamically stable surface nitrates and nitrites is highly favored, ensuring removal of virtually all of the NO.sub.x contained within the gaseous stream undergoing treatment. Third, the invention enables decomposition of the adsorbed NO.sub.x into N.sub.2 and O.sub.2 using a solid-oxide electrolyte in the form of an easily manufactured, compact, self-contained unit. Fourth, only a small direct current (dc) potential (on the order of 1 V) is needed for operation. Finally, the present process eliminates the need for potentially toxic or corrosive scavenging reagents necessary with many current NO.sub.x processing systems. It should also be noted that the invention may be adapted for use in conjunction with adsorption and decomposition of sulfur oxides using substantially the same equipment and procedures.